Heating, Ventilation, Air Conditioning and Refrigeration (HVAC/R) components frequently develop leaks which are sometimes difficult to locate or are not accessible without disassembling all or part of the HVAC/R system. Leaks affect the performance of the HVAC/R system and can even become life-threatening (e.g., CO leaks in a furnace). Similarly, leaks in closed loop plumbing systems are sometimes difficult to locate without disassembly.
Furnaces operate by combusting a gas such as methane or propane, and flowing unconditioned fluid over or through a heat exchanger to heat the fluid. In all cases the combustion gases are separated from the conditioned (heated) building supply air by a normally leak-tight barrier so as to prevent harmful combustion gases and combustible gas byproducts from reaching the habitable spaces. This leak-tight barrier houses the heat exchanger and associated piping and or ductwork and is referred to as the “heat exchanger” herein. The fluid can be a gas (e.g., air) or liquid (e.g., water). The heat exchanger can develop leaks which can decrease the efficiency of the furnace and leak combustion gases and byproducts into the conditioned fluid.
Several methods are known to detect leaks in HVAC/R systems or other closed systems. The methods vary somewhat between vapor compression systems (such as heat pumps, air conditioners, refrigerators, etc.) and furnaces because vapor compression systems have a significant pressure differential, typically 100's of psi (pounds per square inch) pressure differential between the working fluid contained in the system and the environment, whereas in the case of hot air furnaces the pressure difference between the combustion gases and the conditioned air are only a few inches of water pressure (Note: 1 inch or water pressure=0.036 psi). The higher pressure differential in vapor compression systems means that small leaks are typically easier to detect, because the higher pressure differential will force more of any indicator compound through the leak (and make more noise, in the case of acoustic detection).
In general, the leak detection methods range from visual to sensitive electronics. Visual methods include applying a soap solution to the joint or surface where a leak is suspected, and observing for bubble formation caused by the pressure differential between the two regions. This method requires that the technician has access to the suspect area, that a significant pressure differential exists between the two regions where a leak is suspected, and that suspect areas have already been identified. Currently there are furnace leak detectors for natural draft furnaces, as described in U.S. Pat. No. 5,600,057, which rely on spraying a relatively benign fire suppressant (e.g., sodium bicarbonate) into the heated conditioned air stream while observing if there is any change in the flame on the combustion air side of the heat exchanger. If there is any change in the flame's characteristics, usually identified by a color change, then the fire suppressant must be leaking into the combustion side and affecting the flame. While this simple leak detection approach is effective for natural draft furnaces, it is inapplicable for the newer high-efficiency condensing furnaces, because the operator cannot see the flame and therefore cannot see any color change in the flame. Another problem with this method is that the leaks have to be quite substantial to noticeably affect the flame. This technique relies on the pressure difference created by the conditioned air blower to force the liquid fire retardant into the combustion flame which causes a visual difference in the flame temperature, thus changing the flame color.
Yet another visual method is the use of visual or fluorescent dyes. The fluorescent dyes are invisible under ordinary lighting, but visible under ultraviolet (UV) light. The dyes are typically introduced into the vapor compression system when the system is serviced. Leaks are detected by visual inspection of the external surface of the system, and a significant pressure differential between the two regions is necessary to drive the dye though the leak. For fluorescent dyes, a UV light is used to search for dye that has escaped from the system (for example, see U.S. Pat. Nos. 6,327,897 and 6,170,320). The color of the dye when subjected to UV light is normally a bright green or yellow and is easily seen. These methods again require that the technician has access to the suspect area, that a significant pressure differential exists between the two regions where a leak is suspected, and that suspect areas have already been identified.
Other means to detect leaks include the use of compounds which produce a distinct odor. Methane, or natural gas, is sometimes doped with a sulfur-containing gas species which can be easily detected by smell. Ammonia can also be used in this way. These methods are imprecise, and do not allow for pinpointing leaks to any particular area of the HVAC/R system since the odor emanates around the entire HVAC/R system. Some of the compounds used for this method are also toxic if inhaled. U.S. Pat. No. 4,294,716 describes materials which can be used as an odorant for halocarbons and azetropes, primarily chlorofluorocarbons, commonly used as refrigerants in refrigeration, air conditioning and process cooling systems.
Acoustic leak detection is another method. The system is designed to detect very small leaks in pressurized systems, such as vapor compression systems, power boilers, recovery boilers and feedwater heaters. On a boiler, it performs this function by continuously measuring the internal sounds using piezoelectric sensors. The sensors are located in every section of the boiler and number between 12 and 40 per unit depending upon the size of the boiler. The sensor converts the vibrations caused by a tube leak to a voltage, which the system logs and trends. This type of leak detector requires multiple sensors placed throughout the HVAC/R system. The detector system is expensive, and may not be effective in detecting small gas leaks.
Other detection methods utilize a compound which can produce an electronic signal. These types of leak detectors can be placed into one of three categories: nonselective, halogen-selective, or compound-specific. In general, as the specificity of the monitor increases, so does the complexity and cost. In HVAC/R applications, these detectors typically sense the halogens F, Cl, and Br.
In 1963, the Corona Discharge Detection Method (or electrical discharge method), revolutionized the electronic leak detector. A corona is an electrical discharge effect that causes ionization of a molecule. Over the years this concept was refined and improved with extensive research into electrode metals, tip shell materials and finishing procedures as well as circuitry changes and features to improve response time and “clearing” time. One of the most significant advances for this method was the introduction of the “micro-pump” probe assembly. This small motor-driven fan assembly, mounted in the probe handle, actively draws air into the sensing tip. This gives appreciably quicker response time than a system which relies only on diffusion for a gas leak to penetrate the tip, and very much quicker clearing time. These devices were originally designed for chlorine containing compounds, since it is readily detected by corona discharge. Configuring these devices to detect fluorine requires major changes in tip sensitivity and circuit gain since fluorine sensitivity is a factor of 20 or less relative to chlorine.
Sensors such as that described in U.S. Pat. No. 2,550,498 are electrical discharge devices for receiving a sample of an atmosphere containing a concentration of a substance to be detected and comprising cathode and anode elements for producing and collecting ions. U.S. Pat. No. 3,144,600 discloses a halogen leak detector comprising an electrical discharge device sensor with an amplifier of variable gain for amplifying the output current of the sensor. A refrigerant gas leak detector is disclosed in U.S. Pat. No. 3,742,475 that uses a high voltage applied across a pair of electrodes in an atmosphere to generate a continuous corona across the electrodes. In U.S. Pat. No. 3,991,360 a sensor assembly for a halogen gas leak detector which includes a tubular, porous, high purity Alumina (Al2O3) element for supporting electrode and heater components. Another leak detector is disclosed in U.S. Pat. No. 4,282,521 that senses the concentration of gaseous impurities by applying a high voltage across a pair of electrodes to generate a continuous corona current and detecting change in the corona current. Another halogen gas leak detector is disclosed in U.S. Pat. No. 4,488,118 which operates by applying a high voltage across a pair of electrodes to generate a continuous corona current and detecting the presence of halogen gas by sensing changes in the corona current. In U.S. Pat. No. 5,351,037, a refrigerant gas leak detector for detecting the location of leaks of refrigerant gas such as halogen has a sensing tip with a pair of electrodes across which a relatively high voltage is generated to cause a corona current to pass through the electrodes. The voltage applied across the electrodes is varied to maintain the corona current through the electrodes at a substantially constant magnitude. This leak detector includes a gas sensing circuit that detects changes in the concentration of refrigerant gas by sensing the voltage applied to the electrodes.
Heated element detectors also function by using ions from a gas or vapor. These detectors are solid state sensors having the ability of selectively detecting the presence of many gases and vapors within an atmosphere. A solid state element, which contains alkali metal ions for example, which readily accept negative ions (e.g., halogens) of gases and vapors, is brought into reactive contact therewith. The element is prepared to create an outer layer along its boundaries that is depleted of ions. The conductivity of the heated element in an atmosphere free of the reactive gases and vapors is low. However, the presence of one or more of the reactive gases and vapors causes ions to flow across the depletion boundary and increases the conductivity of the element. An electrical circuit then detects an increase in the conductivity of the element, generating a signal indicative of the presence of a reactive constituent in the test atmosphere. U.S. Pat. Nos. 5,932,176, 5,104,513, 5,284,569, 3,751,968, and 3,979,625 describe devices using this principle.
Nonselective detectors are those that will detect any type of emission or vapor present, regardless of its chemical composition. Typical detectors in this category are based on electrical ionization, thermal conductivity, ultrasonics, or metal-oxide semiconductors. These detectors are typically quite simple to use, very rugged, inexpensive (normally less than $500), and almost always portable, thus making them ideal for leak pinpointing applications. However, their inability to be calibrated, long-term drift, lack of selectivity, and lack of sensitivity (detection limits usually between 50 and 100 ppm for 1,1,1,2-tetrafluoroethane (hydrofluorocarbon 134a, or HFC-134a)) limit their use for area monitoring.
Halogen-selective detectors use a specialized sensor that allows the detection of compounds containing the halogens fluoride, chloride, bromide, and iodide without interference from other species. The biggest advantage of these detectors is a reduction in the number of false alarms caused by the presence of other compounds with the halogen-containing refrigerant. These detectors are typically easy to use, feature higher sensitivity than the nonselective detectors (detection limits are typically <5 ppm when used as an area monitor and <0.05 oz/yr when used as a leak pinpointer), and are very durable. In addition, due to the partial specificity of the detector, these instruments can be calibrated easily. These types of detectors are commonly used by HVAC/R technicians, and are relatively inexpensive and compact. Typically, these halogen leak detectors for halogen-containing compounds function indirectly or directly from the ionization of halogen-containing compounds, and the types of sensors currently used include corona discharge or heated elements.
The most complex and expensive detectors are compound-specific detectors. These detectors are typically capable of detecting the presence of a single species without interference from other compounds. Compound-specific detectors typically are infrared-based (IR), although some of the newer types are infrared-photoacoustic based (IR-PAS). For example, see U.S. Pat. No. 6,791,088. The IR and IR-PAS detectors normally have detection limits around 1 ppm, depending upon the compound being detected. There are also several IR detectors on the market that have detection limits of approximately 10 ppm. These detectors typically have a much lower price per unit and are less complex than those with lower detection limits. For refrigerants other than 2,2-dichloro-1,1,1-trifluoroethane (hydrochlorofluorocarbon 123, or HCFC-123), these units probably will yield acceptable performance. Due to recent improvements in technology, the price of the compound-specific detectors has dropped by about 50 to 60% during the last year. For most of 1991, IR-based detectors could be purchased for approximately $10,000 per unit. In 2005, units with comparable performance are available for only $3,500 to $4,000.
The sensitivity of a device is determined by a number of factors. The most important factors for leak detection are the method of detection and the material being detected. For example, a halogen leak detector that demonstrates high sensitivity for dichlorodifluoromethane (chlorofluorocarbon 12, or CFC-12) may have worse sensitivity for HCFC-123 and very poor sensitivity for HFC-134a. Sensitivity differences of 20-1000× have been reported when comparing CFC-12 to HFC-134a with some halogen leak detectors. In this case, the variations in sensitivity would be due to less chlorine, which is very easily ionized and detected. The bond dissociation energies of CH3—Cl and CH3—Br are 22% and 35% less than that of CH3—F, which further illustrates the relative ease by which Cl and Br can be dissociated from organic molecules compared to fluorine, making their detection much easier. Compounds which contain more atoms of fluorine per molecule and are more easily broken down in a halogen leak detector would also provide enhanced sensitivity, especially for more easily dissociated compounds which ONLY contain fluorine and not Cl and/or Br.
Detection limits for monitors are measured in two ways: oz/yr for pinpointing applications and ppm for area monitoring. Portable leak pinpointers typically have detection limits reported around 0.25 oz/yr, while area monitors have detection limits as low as 1 ppm, although a more typical value is 3 to 4 ppm for most compounds.